When adjustment power of the crystalline lens becomes weak in presbyopia, accommodation for seeing a close object becomes impossible. In this case, generally, spectacles having convex lenses may be used to compensate for the adjusting power.
An example of known convex lenses for presbyopia is shown in FIG. 5. This is a lens 50 for presbyopia with a front surface 50F having a radius of curvature r(1) smaller than a radius of curvature r(2) of a rear surface 50R thereof.
Specifically, take a lens of 2D for example, where the smaller radius of curvature r(1) is set to 116.754 mm, and the larger radius of curvature r(2) to 218.667 mm.
In the illustrated spherical lens for presbyopia, usually the ratio of size (lateral magnification) between an object and a virtual image seen through the lens 50 varies with the height of the object. That is distortional aberration. This phenomenon is the more salient the higher is the diopter of the lens.
In addition, as shown in hatching in FIG. 5, the conventional lens 50 provides a range of distinct vision. When, for example, the user or patient wears lenses of 2D, a corrected near point on 02 optical axis a is at a distance of 300 mm (which is a distance from the front surface of the cornea in an eyeball 52), and a corrected far point is at a distance of 504 mm (the focal length of the lens) assuming that the far point is at infinity for the naked eye of the user. The range of distinct vision exists between the near point at 300 mm and the far point at 504 mm for all looking directions of the eye. In FIG. 5, angle 01 or 02 forms 30 degrees with the optical axis a. The range of distinct vision becomes small when the diopter of the lens becomes large.
Thus, when the conventional spherical lens 50 is used, the range of distinct vision is limited to narrow regions, and an image becomes deformed by the distortional aberration. This results in the disadvantages of the eyes becoming fatigued after a long period of use in the absence of a comfortable visual sense.